EP0417409B1 - System for the opto-electrical command of a flame cutting machine - Google Patents

Description

The invention relates to a device for the optical-electronic control of a cutting machine according to the preamble of claim 1.

It is already known to continuously control a gas cutting process via a photoelectric element directed at the cutting flame by detecting a change in the brightness of part of the gas cutting with an oxidizing reaction using the photoelectric element (DE-AS 22 03 194). The gas cutting process is thus intended to be carried out by automatically controlling the cutting speed and by stopping the cutting device, e.g. in the event of a misfire. In particular, the photoelectric element is attached to the upper end of a burner and is aligned with the bright part of the flame via a central bore for the cutting oxygen in the burner and via a channel provided in the cutting nozzle of the burner. The cutting speed is continuously controlled by detecting the change in the output voltage of the photoelectric element within a predetermined range. However, this photoelectric element detects only one criterion, that is, the change in the brightness of the gas cutting part with an oxidizing reaction, to stop the gas cutting operation when a misfire occurs and to control the cutting speed.

In order to increase the performance of flame cutting and to save material, it has already been proposed to separately record and evaluate the temperature of the flame, the ignition temperature of the metal to be cut and the temperature of the face cut surface during cutting. A signal corresponding to the flame temperature is used Control of the gas supply evaluated, a signal of the ignition temperature of the metal for controlling the cutting oxygen and for the movement of the beam evaluated and the signal of the temperature of the end face for the command output to interrupt the cutting process evaluated.

For this purpose, in the case of the proposed optical-electronic control, a photo transmitter accommodated in the burner with the channel of the cutting oxygen consists of three individual photo elements which are connected to the evaluation device independently of one another. The photo elements have different spectral sensitivities. A first photo element is connected to a control system for the supply of gas and for coordinate drives and its optical characteristic (sensitivity) should correspond to the temperature of the flame. A second photo element is connected to a control system for the supply of the cutting oxygen and for the coordinate drives; its optical characteristic should correspond to the ignition temperature of the metal. A third photo element is also connected to the control system for the cutting oxygen and for the coordinate drives, but its optical characteristics should be adapted to the temperature of the face cutting surface. The purpose of adjusting the optical characteristics of the three photo elements was only to maximize their sensitivity to the optical phenomenon detected by the respective photo element. The output signals of the photo elements were only evaluated separately and with regard to their amplitude, ie the brightness of the flame or the workpiece, in the spectral sensitivity range of the respective photo element. However, the evaluation of the amplitudes of the output signals does not provide any clear information about the existence of the monitored flame, the reaching of the ignition temperature or the cutting of the torch when the photo elements are installed in the cutting torch, because the amplitudes of the output signals of the Photo elements are subject to strong interference. Disruptive changes in the amplitude level occur when changing the nozzle, changes in the height of the torch above the workpiece and variations in the cutting speed.

In a known device according to the preamble of claim 1 for cutting sheet metal, the fuel gas and the oxygen supply as well as other parameters are set automatically depending on temperatures which are detected with two sensors at different locations in a welding gap (US-A-4 439 249). In particular, the surface temperature at the shoulder of the welding gap and the temperature in the welding gap are sensed, the frequency of the light emitted in the welding gap being measured in addition to the brightness. The sensors used for this purpose are arranged on the outside of the burner, but one of the two sensors can also be located in a channel of the burner. In general, any conventional type of temperature sensor, but preferably infrared sensors, can be used. The outputs of the sensors are connected to a microprocessor. The temperatures recorded at the two measuring points are thus evaluated with logic operations, in particular AND operations. There are no special measures to suppress interference.

The state of the art of recording light emissions during gas-tungsten arc welding in order to improve the weld pattern includes the use of a monochromator, in particular a double monochromator, with which the entire spectrum of the light emitted during the welding process is recorded (WELDING JOURNAL, vol. 66, no. 12, December 1987, MIAMI US, pages 369a - 377s; EW KIM et al .: "Visible light emissions during gas tungsten arc welding and its application to weld image improvements."). Such a monochromator is a complicated and complex physical device, the use of which in the operation of a flame cutting machine is not seriously considered. Accordingly, the use of a monochromator for direct control of a cutting torch is not disclosed.

The use of several sensors of different spectral sensitivity for optoelectronic control of oxygen cutting has been proposed, but no quotient of the output variables of the sensors is formed (EP-A-0 408 755). Rather, each of the sensors is connected via an amplifier and its own associated signal generator to an electrical control system which triggers operations of the cutting machine as a function of the brightness of the measuring points detected with each sensor. However, the detection of only the respective brightness is not immune to interference.

The invention is therefore based on the object of further developing a device of the type mentioned at the outset such that the output signals of the photoelectric sensors automatically generate clear criteria for the formation of control and monitoring signals without being exposed to falsifying interference. The compact arrangement of the photoelectric sensors in the cutting torch should be retained if possible.

This object is achieved by the design of the device for optical-electronic control with the means specified in the characterizing part of claim 1.

It is important here that no longer any photoelectrically monitored phenomenon of flame cutting is assigned to one of several photoelectric sensors with different spectral sensitivity, the output values of which are evaluated separately, but that all processes of interest are preferably recorded with a pair of photoelectric sensors with different spectral sensitivity, the Output signals in electronic means for The formation of quotients is set in relation to one another, so that the output signals are no longer a measure of the brightness of the area detected by the photoelectric sensors, but rather correspond to its (color) temperature, and are preferably proportional. As a result, influences which influence the amplitudes of the output signals emitted by the photoelectric sensors uniformly have no disruptive effect on the output signals and their evaluation.

In order to obtain clear control and monitoring signals for the different phases or states, according to claim 3, discriminators for different evaluation ranges of this quotient are provided in the evaluation device, to which the quotient of the output variables of the sensors are applied, of which in each case one evaluation range depending on the occurrence of an external control command can be activated. An evaluation area is therefore assigned to each phase of flame cutting, so that a clear statement relating to this phase can be obtained from the wavelength of the light received in this phase, which is determined by the sensors and the formation of the quotient, and which is used for further control of flame cutting. In detail, these phases or evaluation areas are the burning of the (preheating) flame, the course of the preheating of the workpiece and the actual cutting process with the supply of cutting oxygen. Depending on the intended field of application of the device, these and possibly other phases can be determined and implemented by using a discriminator in the evaluation device.

As transmitters of the external control commands for activating the discriminators or evaluation areas in the evaluation device, such transmitters are provided according to claim 4, which issue control commands for igniting the flame, preheating the workpiece and cutting while supplying cutting oxygen. Such sensors are generally part of the basic equipment for the control of a cutting machine, so that they do not increase the effort in connection with the present invention; rather, these sensors only have a new, additional function.

Particularly suitable for accommodating the photoelectric sensors in the head of a burner, since it is very compact, is an opto-double diode on a common substrate. The diodes of the opto-double diode have different sensitivity maxima depending on the light wavelength. Such a monolithic double diode is known under the name PD 153 (Sharp, Japan). It essentially consists of two diodes of different spectral sensitivity with a common cathode. The two anode connections and the common cathode connection are led out of the double diode. The light sensitivity maximum of one diode is slightly below 600 nm, while the light sensitivity maximum of the other diode is 900 nm. - The combination of two sensors with the different spectral sensitivity given above results in a relatively large evaluable range of the received light wavelength.

In a variant of the pair of sensors with different spectral sensitivity, similar opto-diodes, i.e. those with the same spectral sensitivity are used if, according to claim 6, one of the two opto-diodes is arranged in a reflection beam path and another of the two opto-diodes is arranged in the transmission beam path of a partially transparent mirror which has a spectrally different reflection and transmission. In particular, the mirror for the present application can advantageously be such that it transmits the same part of the luminous flux and reflects at a light wavelength of 700 nm ("50% edge").

In yet another embodiment of the two sensors of different spectral sensitivity, these can each consist of a separate opto-diode with different spectral characteristics or sensitivity.

As such opto-diodes of different spectral characteristics can be optical in particular according to claim 8 Germanium diode and a silicon diode may be provided.

It is essential in all embodiments that, by forming the quotient of the output variables or output signals of the two sensors of different spectral sensitivity, a resulting signal is independent of the absolute amount of light with which both sensors are exposed and has only a dependency on the received light wavelength.

The electronic means for forming the quotient can be simplified by at least one sub-emitter according to claim 9, in which the quotient of the output variables of the sensors of different spectral sensitivity is approximately formed.

In a further development of the latter device, each of the two inputs of the subtractor can be preceded by a logic amplifier, which is acted upon by one of the output variables of the sensors of different spectral sensitivity in order to achieve a more precise quotient formation in connection with the subtractor.

In order to form a defined control or monitoring signal from the output signals or output quantities of the sensors after the formation of the quotient or difference, at least one comparator is provided as a discriminator in the evaluation device. The discriminator emits a defined output signal depending on whether the sensors are exposed to light above or below a threshold wavelength. The limit wavelength is determined by the reference signal, in particular a reference voltage, with which an input of the comparator is applied.

In order to define an evaluation range within which a monitored process or a phase of cutting is in the target range, a window comparator is used in the evaluation device as a discriminator, which comprises two comparators, one of which each has an upper limit and one The lower limit of the evaluation range or target range that can be activated can be preset. The outputs of the comparators are linked to one another via a logic element in order to emit a control or monitoring signal or readiness signal if the output values of the comparators are within the target range. The limits of the target range can be set with the reference signals or reference voltages on the comparators, whereby the upper limits and the lower limit can be set individually.

The features specified in claim 13 are provided for monitoring several cutting torches of a flame cutting machine. It is important here that a complete evaluation device with discriminators or window comparators for the individual evaluation areas is not assigned to each pair of photoelectric sensors with different spectral sensitivity, but that only one set of discriminators or window comparators are used multiple times in time-division multiplexing, namely for all pairs of sensors. The corresponding device not only includes multiplexers at the input and output of the evaluation device, but also an addressable register which assigns the control and monitoring signals formed in the evaluation device to the individual cutting torches in a preprogrammed sequence.

Fig. 1

einen Schneidbrenner, in dem die fotoelektrischen Sensoren untergebracht sind, in einer Seitenansicht, teilweise geschnitten,

Fig. 2

schematisch die beiden Sensoren unterschiedlicher spektraler Empfindlichkeit als monolitische Opto-Doppeldiode,

Fig. 3

einen typischen Verlauf der spektralen Empfindlichkeit der beiden Dioden der Opto-Doppeldiode nach Fig. 2 in Abhängigkeit von der Wellenlänge des Lichts,

Fig. 4

das Verhältnis der Ausgangsgrößen, nämlich der Ausgangsströme der beiden Dioden nach Fig. 2 ebenfalls in Abhängigkeit von der Wellenlänge des Lichts,

Fig. 5

eine schematische Darstellung der Anordnung der beiden Sensoren mit einem teildurchlässigen Spiegel,

Fig. 6

die Transmissions-Reflexionskennlinie des teildurchlässigen Spiegels in Abhängigkeit von der Lichtwellenlänge,

Fig. 7

die Ausgangsgrößen, nämlich Ausgangsströme der beiden Sensoren gemäß Fig. 5,

Fig. 8

eine Auswerteeinrichtung in Blockdarstellung,

Fig. 9

die Auswertebereiche in Abhängigkeit von bestimmten Phasen des Schneidvorgangs einschließlich vorbereitungsphasen,

Fig. 10 - 13

Blockschaltbilder von vier Ausführungsformen der Einrichtung zur optisch-elektronischen Steuerung der Brennschneidmaschine.

The invention is explained below with reference to a drawing with 13 figures. Show it:

Fig. 1

a cutting torch, in which the photoelectric sensors are housed, in a side view, partially cut,

Fig. 2

schematically the two sensors of different spectral sensitivity as a monolithic opto-double diode,

Fig. 3

2 shows a typical course of the spectral sensitivity of the two diodes of the opto-double diode as a function of the wavelength of the light,

Fig. 4

the ratio of the output variables, namely the output currents of the two diodes according to FIG. 2, also as a function of the wavelength of the light,

Fig. 5

1 shows a schematic representation of the arrangement of the two sensors with a partially transparent mirror,

Fig. 6

the transmission-reflection characteristic of the partially transparent mirror as a function of the light wavelength,

Fig. 7

the output variables, namely output currents of the two sensors according to FIG. 5,

Fig. 8

an evaluation device in block diagram,

Fig. 9

the evaluation areas depending on certain phases of the cutting process including preparation phases,

Figures 10-13

Block diagrams of four embodiments of the device for the optical-electronic control of the flame cutting machine.

In Fig. 1, a cutting torch, generally designated 1, is shown, the upper connection part or head 2 of which is cut in the longitudinal direction in the position of use.

In the extension of a cutting oxygen channel 3, an opto-double diode 4 is accommodated in the upper connection or head part 2, which is preceded by an imaging lens 5. The lens 5 serves to image the area that lies in front of or below an opening (not shown) of a nozzle 6 on the opto-double diode. The area of interest is the flame emerging from the nozzle 6 during operation of the cutting torch or the part to be machined (cut) of the workpiece not shown below.

2 shows that the monolithic optical double diode consists of a first diode 7 and a second diode 8, which have a common cathode connection 9 (a cathode). Anode connections are labeled 10 and 11.

FIG. 3 shows the relative spectral sensitivity of the first diode with the line 12 and the relative spectral sensitivity of the second diode in FIG. 2 with the line 13 as a function of the wavelength of the light that strikes the two diodes. It can be seen that the first diode line 12 has a maximum near the wavelength 600 nm, while the second line 13 second diode has a maximum near 900 nm.

If the ratio of the outputs of the second diode 8 to the first diode 7 in FIG. 2 is formed, namely that Ratio of their short-circuit currents, the dependence on the light wavelength is as shown in FIG. 4. It can be seen that the ratio of the output currents or output quantities of the two diodes is clearly related to the light wavelength.

Instead of the opto-double diode 4, a sensor arrangement according to FIG. 5 can be accommodated in the connection or head part 2 of the cutting torch, but with a larger space requirement. This sensor arrangement essentially consists of a partially transparent mirror 14, in the transmission beam path of which there is a first opto-diode 15, while in the reflection beam path of which a second opto-diode 16 is arranged. The transmission-reflection characteristic of the partially transparent mirror is given in FIG. 6. The "50% edge", at which the same amount of light is transmitted through the mirror and is reflected, is 700 nm. With longer-wave light, more is detected, below which more light is transmitted through the mirror.

This results in a dependency of the output quantities or output currents of the two opto-diodes 15 and 16 according to the lines 17 and 18 as a function of the light wavelength as shown in FIG. 7. If the ratio is formed from the two output currents i 1 and i 2 of the opto-diodes 15 and 16, a similarly defined characteristic curve can be set as shown in FIG. 4 to the opto-double diode.

The evaluation of the output variables of the two sensors or opto-diodes or the opto-double diode can therefore be carried out in basically the same way with the evaluation device 20 shown generally in FIG. 8.

Regardless of whether the output currents i₁ and i₂ of the separate opto-diodes 15 and 16 are to be evaluated or the output currents of the opto-double diode 4, these output currents first reach a quotient image 19 according to FIG. 8, in which the ratio of the two currents at least is approximately formed. According to FIG. 4, this quotient is a measure of the light wavelength received. In order to form unambiguous control or monitoring signals from this quotient, a distinction is made in the evaluation device 20 as to which phase, including the preparation phase for cutting, is the current light wavelength measurement. For this purpose, control commands, which are generated to ignite the preheating flame, to run the preheating process and to initiate cutting by supplying cutting oxygen in a conventional control device of a flame cutting machine, are fed to the evaluation device via its control input 21. A selection of the current evaluation range for the respective operating phase can then be made in the evaluation device by discriminators (not shown in FIG. 8 but to be discussed further below). This enables a clear statement of the quotient of the currents i 1 and i 2.

This relationship is shown graphically in FIG. 9. The various operating states of the flame cutting machine or the respective cutting torch are indicated in the ordinate axis: off, burning the flame, preheating the workpiece, cutting while supplying cutting oxygen. These include evaluation areas 22, 23, 24, 25, in the inner hatched part of which the quotient i₂ to i₁ is in a desired range and signals the proper functioning of the cutting torch in the respective operating phase. An output 26 of the evaluation device accordingly outputs control and monitoring signals which indicate whether the preheating flame is actually burning, whether the ignition temperature has been reached and whether the Cutting torch cuts.

In the first embodiment of the electronic means for quotient formation and the evaluation device according to FIG. 10, each of the two currents ID1 and ID2, which correspond to the currents i 1 and i 2 in the preceding figures and which are generated, for example, by the opto-double diode 4, in a part 27, which is delimited by broken lines, fed to the circuit arrangement. Each of the two currents is converted in a current-voltage converter 28 or 29 into a proportional voltage, which is amplified in each branch with a logic amplifier 30 or 31. The voltages amplified in accordance with the characteristics of the logic amplifiers are subtracted from one another in a subsequent subtractor 32. The output voltage of the subtractor is thus approximately the quotient of the input currents ID1 and ID2 and is therefore a measure of the light wavelength received by the opto-double diode. The part 27 of FIG. 10 thus corresponds in effect to the quotient image.

The output variable of the quotient generator or the output voltage of the subtractor 32 is fed into an input 33 of a comparator 34, the second input 35 of which is supplied with a reference voltage. The comparator distinguishes whether the voltage at the input 33 corresponding to the light wavelength is above or below the reference voltage by a predetermined value and accordingly forms a control or monitoring signal at its output 36. The evaluation device here therefore consists only of a comparator for emitting a control - or monitoring signal.

The second embodiment according to FIG. 11, in which the same components as in FIG. 10 are provided with the same reference numerals, differs from the first embodiment only in the details of forming the quotient in one Part 37. The current-voltage converters 28, 29 are each followed by a smoothing low-pass filter 38 or 39, the output voltage of which passes through a simple voltage follower 40 in one branch and an inverting voltage follower 40 in the other branch. By interconnecting the outputs of the voltage followers 40 and 41 Resistors 42 and 43 then form a difference between the output voltages of the voltage followers, which as a result represents an approximate quotient formation of the current quantities ID1 and ID2 of the opto-double diode 4. This signal representing the quotient runs into the input 33 of the comparator 34. The second input 35a of the comparator is grounded here. The limit wavelength at which the comparator switches the output variable is determined here by the ratio of the resistors 42 to 43.

In the third embodiment of the device according to FIG. 12, there are similarities to the second embodiment according to FIG. 11 essentially up to and including part 37a, which corresponds to part 37 in FIG. 11. The outputs of the voltage followers 40 and 41 are not only connected to one another in FIG. 12, but via a more complex resistor arrangement. An interconnection of two resistors, as in FIG. 11 of the resistors 42, 43, is present in the third embodiment in each of the potentiometers 47-52, a wiper of each of the potentiometers being guided to an input of one of the comparators 53-58. The other input of each comparator is here at zero potential. A limit wavelength at which a signal change of the output signal of the respective comparator occurs can thus be set on each comparator, as explained in connection with the resistors 42, 43 in FIG. 11. Two comparators each, eg 53, 54; 55, 56; 57, 58; are connected together on the output side to form a window comparator 44 or 45 or 46. The outputs of the Both comparators, which form a window comparator, are linked by a logic element, which can be designed in particular as an AND gate. In detail, the outputs of the comparators 43, 44 are connected via the logic element 59, the outputs of the comparators 55, 56 via the logic element 60 and the outputs of the comparators 57, 58 via the logic element 61 Comparators, e.g. 53, a window comparator is assigned to the upper limit of an evaluation range, which is set by the potentiometer 47, and a comparator 54 of the lower limit of the evaluation range, adjustable by the potentiometer 48, thus form both comparators 53, 54 in connection with the Logic element 59 the window comparator 44, which forms a window with an upper and a lower limit, within which an output signal of the logic element assumes a first value and outside the limits of which the output signal adjusts itself to a second value. In this way, the window comparators 44, 45, 46 each generate output signals for the “flame on” or “piercing” or “cutting” state. Each of these states or phases corresponds to an evaluation range with the limits that can be set using the potentiometer.

In the embodiment according to FIG. 13, not only is one cutting torch 1 monitored and controlled, but rather a plurality thereof, including the cutting torches 62, 63, 64. Each of the cutting torches is equipped with two sensors of different spectral sensitivity, in particular an opto-double diode. The output signals of the opto-double diodes, namely their currents, in turn, as described in particular in connection with FIG. 11, are converted into proportional voltages in a current-voltage converter and smoothed in a low-pass filter. The corresponding units of the current-voltage converter and the low-pass filter are designated 65-68 in FIG. The output signals from these units 65 - 68 are not processed in parallel, but rather staggered over time in the multiplex process. For this purpose, the output currents of an opto-double diode or two diodes of a pair act on a voltage follower 40 and an inverting voltage follower 41 via a first multiplexer 69, as described in connection with the second embodiment according to FIG. 11 with reference to a cutting torch or a sensor arrangement . The evaluation of the output signals of the voltage followers 40 and 41 is then in three window comparators 44, 45, 46, as was explained in detail in connection with the third embodiment in FIG. 12. In addition, a limit value check is carried out with a comparator 70, one input of which is supplied with the inverted voltage from the inverting voltage follower 41 and the second, not designated input of which can be set to a minimum voltage via a potentiometer 71, at which a switchover takes place. The set resistance value of the potentiometer is thus used for limit value control. The outputs of the window comparators 44, 45, 46 to be switched over for emitting monitoring and control signals of the cutting torch in each case being monitored in accordance with the setting of the first multiplexer 69 are fed into a second multiplexer 72. The switching of the multiplexers takes place synchronously controlled by a clock generator 73 and an address counter 74 and, as far as the second multiplexer 72 is concerned, via an addressable register 75. For this purpose, the addressable register is connected to the second multiplexer 72 via a line 76. The further control of the second multiplexer 72 is carried out depending on a range selection on lines 77 and 78 and on the output signal of the comparator 70 on a line 79. The evaluation areas are ultimately activated here by the second multiplexer 72. The assignment of the selected evaluation area to one of the cutting torches 1, 62-64 then takes place via the addressable register 75 In this way, several cutting torches or cutting points can be reliably monitored and controlled with relatively little effort.

Claims (14)

1. System for the opto-electric command of a flame-cutting machine with photoelectric sensors (4, 7, 8) of varying spectral sensitivity which are directed operationally at a workpiece to be cut and which on the output side are connected to an evaluation unit (20) in order to form control signals particularly for the feed of the flame cutter (1) in relation to the workpiece, characterized in that at least two of the sensors (4, 7, 8) of varying spectral sensitivity are directed to the same area of a flame and of the workpiece to be cut and in that electronic means (27) are provided for determinging the proportion which are connected to outlets of the two sensors (4, 7, 8) of varying spectral sensitivity in such a way that the proportion of the output values of the sensors (4, 7, 8) is determined and in that the means for determining the proportion on the output side are connected to the evaluation unit (20) in such a way that at least one signal is generated which simply corresponds to the colour temperature of the detected area of the flame and of the workpiece to be cut.
2. System according to claim 1, characterized in that the photoelectric sensors (4, 7, 8) of varying spectral sensitivity are disposed in a flame cutter (1) of the flame-cutting machine.
3. System according to claim 1, characterized in that discriminators (window comparators 53-58) for varying evaluation ranges of the proportion of the output values of the sensors are provided in the evaluation unit, of which in each case an evaluation range can be activated in accordance with the generation of an external control command.
4. System according to claim 3, characterized in that transmitters are connected to the evaluation unit (20) for igniting the flame, for preheating the workpiece and for cutting and in that discriminators for an ignition range, a preheating range and a cutting range of the proportion of the outputs of the sensors are provided in the evaluation unit.
5. System according to one of claims 1 to 4, characterized in that the sensors of varying spectral sensitivity are formed by a opto-double diode (4) on a common substrate so that the diodes (7, 8) of the opto-double diode have varying maximum sensitivity values in accordance with the light wavelength.
6. System according to one of claims 1 to 4, characterized in that the sensors of varying spectral sensitivity consist of two similar opto-diodes (15, 16), of which one opto-diode (16) each is disposed in a reflection ray path and one opto-diode (15) is disposed in the transmission ray path of a semireflecting mirror (14) and in that the semireflecting mirror has a different reflection and transmission from a spectral point of view.
7. System according to one of claims 1 to 4, characterized in that the sensors of varying spectral sensitivity consist in each case of a separate opto-diode of varying spectral characteristic (sensitivity).
8. System according to claim 7, characterized by one optical germanium diode and silicon diode each as sensors of varying spectral sensitivity.
9. System according to one of the preceding claims, characterized in that the electronic means for determining a proportion are provided through at least one subtracter (32), in which the proportion of the output values of the sensors of varying spectral sensitivity is determined by approximation.
10. System according to claim 9, characterized in that one logic amplifier (30, 31) is, in each case, connected in series to each of two inputs of the subtracter (32) so that the logic amplifiers are activated with, in each case, one of the output values of the sensors of varying spectral sensitivity.
11. System according to claim 1, characterized in that at least one comparator (34) is provided as a discriminator in the evaluation unit with the comparator giving off, in each case, a defined output signal according to whether the sensors are activated with light above or below a boundary wavelength.
12. System according to claims 1, 3 and 11, characterized in that a window comparator (44, 45, 46) is provided, in each case, as a discriminator in the evaluation unit which comprises two comparators (e.g. 53, 54), of which, in each case, one can be preset to an upper limit and to a lower limit of the evaluation range which can be activated and the output values of which can be connected to each other via a logic element (59, 60, 61) for giving off a monitoring signal (ready signal) or control signal.
13. System according to claim 12, characterized in that a pair of photoelectric sensors of varying spectral sensitivity are disposed in each flame cutter for monitoring several flame cutters (1, 62-64), that the pairs of photoelectric sensors are connected via a first multiplexer (89) to inputs of the window comparators (44, 45, 46) of different evaluation ranges, that the window comparators are connected on the output side to a second multiplexer (72) which is provided with control inputs for activating the various evaluation ranges in accordance with the external control signals and the output of which is connected to an addressable register (75) which gives off control and monitoring signals for the various flame cutters and in that the multiplexer (69, 72) and the addressable register (75) are synchronized with each other.
14. System according to one of the preceding claims, characterized in that a maximum sensitivity value of one of the two sensors is about 900 nm and that a maximum sensitivity value of the other of the two sensors is about 600 nm.

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